The classical Josephson junction is an electronic device in which two superconducting elements are separated by a thin insulating film. The two superconducting elements can be of the same composition, or may differ from each other. The unique current vs voltage characteristics of the Josephson junction depend on the binding energy of paired charge carriers in the superconducting elements. When both elements are the same, the Josephson junction acts as a bidirectional diode, switching from an essentially non-conducting state (with very little leakage current) to a conducting state. The switching occurs at a voltage corresponding to the binding energy of the charge-carrier pairs. When the two superconductors are unequal, or their charge carriers possess different binding energies, a more complex characteristic results in which a region of negative resistance exists.
Until recently, it was believed that superconductivity above 23K was not possible. This belief was noted in the theoretical work now named the BCS theory (Bardeen, Cooper and Schrieffer) which predicted such an upper limit.
The temperature at which superconductivity occurs in a superconductor (in the absence of any external magnetic fields) is termed the critical temperature of that superconductor and this term will be used herein.
In the early 1970's a number of theoretical proposals were presented, suggesting that the critical temperature for superconductivity could be increased. (V. L. Ginzburg, Usp. Fiz. Nauk. 101, 185 (1970)) (D. Allender, J. Bray, J. Bardeen, Phys. Rev. B8, 4433 (1973)), but the lack of any discoveries of superconductivity above 23K, solidified the belief that indeed this critical temperature could not be exceeded. A significant experimental breakthrough in high temperature superconductivity (critical temperatures in excess of 23K) was provided in November 1986 by Bednorz and Muller when they published a tentative disclosure of high temperature superconductivity (Georg Bednorz and Alex Muller, Z. Phys. B64, 189 (1986)), followed rapidly by confirmation by others.
Confirmation of a critical temperature of 93K was reported by Chu for yttrium-barium-copper oxide ceramic (M. K. WU, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Phys. Rev. Lett. 58, Mar. 2, 1987, p. 908.) This material was dubbed the 123 compound and served as a model for advanced research in the field.
During 1987 and 1988, a number of families of high temperature superconductors were discovered with confirmed critical temperatures all the way to 162K. These materials are usually ceramics containing copper (including elements in the trivalent state), an alkaline metal (Ca, Sr. or Ba) and a rare earth including Yttrium.
In classical low-temperature Josephson junctions, the insulating layer is traditionally obtained by an oxidation step on the metallic superconducting surface, thus creating an insulating oxide. In many instances, deposition of an insulating oxide is practiced as well. The thickness of the insulating layer is well controlled and is, in some cases, in the neighborhood of 100 to 1000 angstroms in thickness.
In the high temperature superconductors (herein high temperature superconductor means a superconductor with a critical temperature above the BCS limit of 23K) which have now been synthesized, however, the coherence length of the charge carriers pairs is of the order of 20 to a maximum of 100 angstroms. Thus for a Josephson junction to operate in the mode whereby paired charge carriers tunnel through the junction with the associated tunneling effects, insulating layers that have thicknesses of the same order of magnitude as the coherence length are mandatory. Furthermore, the high temperature superconducting substances discovered so far are all oxides themselves, unlike the metallic classical superconductors.
There have been a number of natural major road blocks to achieving a very thin insulating barrier with traditional oxides. Specifically, there are problems of interdiffusion and difficulties in obtaining thin layers of insulators by current deposition practices. Most of the high temperature superconductors are mixed oxides, and are relatively unstable from a thermodynamic point of view. Furthermore, in any deposition process which one may employ to form said insulating layer, the diffusion coefficient of the various cations in the superconducting oxide and the insulating oxide varies between the cations. As a result of these differences, disproportionation (from the original composition) of the cations in the superconductor near the insulating layer often occurs. This is associated with a loss of superconductivity near such insulating layer, thus defeating the purpose of the junction.
Non-superconductive phases arise when the original oxide superconducting phase comes into contact with other oxides at high temperatures (above 700.degree. C. for most superconducting compounds, but even lower for others). These non-superconducting layer will form with depths that are greater than the required depth of the insulating layer in the high temperature Josephson junction, thus defeating the intended purpose of the device.